In order to obtain high frame rates on CCD image sensors it has become necessary to read out the image in parallel through multiple outputs. For a sensor with two outputs there are two different regions of the image which pass through separate signal processing electronics. Minor process variations in the fabrication of the CCD cause the output amplifiers to have slightly different gains, typically 1% or less. More significantly, the signal processing electronics (SPE) external to the CCD contain many different components. The sum of the variations in all of the SPE components contributes to overall nonlinearity and offset errors which are easily visible in the image.
The degree to which gain and offset errors degrade the image quality depends heavily on the complexity of the image as well as whether or not the sensor is a color or monochrome device. If the image being viewed is very flat and uniform, such as a white piece of paper, then signal errors as small as 1% between outputs are easily visible. The problem is exacerbated by color devices because the blue pixels usually have a lower photoresponse than red or green pixels. Due to the large gain needed on blue pixels, the eye can spot signal differences less than 1%. Subtle changes in color across a block boundary are also easy to see.
As can be seen by the foregoing discussion, there remains a need within the art for a method and apparatus that can properly balance image sensors having multiple output block format. The present invention, as disclosed herein, will describe and evaluate a new method and apparatus for eliminating image imbalances on multiple output image sensors.